Recent Developments on Rotaxane-Based Shuttles: An Update to 2010

This update reviews the major developments on rotaxane-based shuttles during the period of August 2008 to October 2010 and is organized similarly to the previous review [1]. The progress in the field of chemical controlled shuttles can be summarized as a significant increase of the shuttling rates, second-generation pH-switchable Pd(II)-complexed rotaxanes, and new synthetic approaches, especially those based on Diels-Alder cycloadditions. The new synthetic routes allow ready access to rotaxanes that feature the template site fully incorporated into the interlocked product. This template site is unusual for active-template reactions and produces shuttles that exhibit entropy-driven translational isomerism with remarkably improved positional discrimination as was shown by the incorporation of a hydrogen-bonding TEG-station into an imine-bridged rotaxane. With respect to photochemically powered molecular switches, the most important result originated from the synthesis of a degenerate, donor-acceptor, light-gated, STOP-GO molecular shuttle. For electrochemically controllable bistable rotaxanes, the first example of a molecular shuttle with three different stations, which dramatically increase the shuttling process rate, has been reported. Finally, the last study in this review involves redox-driven switching and electrochromic responses of LC films containing a bistable [2] rotaxane. The presence of a polymer electrolyte provides new prospects for the further development of rotaxane-based molecular materials by exploiting the dynamic, anisotropic, and coherent properties of liquid crystals.